To improve process control for some semiconductor manufacturing processes, integrated metrology (IM) modules are attached to the process tool and used to measure and quickly provide feedback for real-time control of the process. A typical IM module is built with the same form factor as a loadport, allowing it to be attached to the process tool EFEM (equipment front end module) in place of a loadport. In this case, the wafer handling robot, which is a part of the EFEM, can easily load wafers into the IM module. Typically, there is an open port between the IM and the EFEM, allowing the robot to freely load wafers into the IM.
In order to fit into the form factor of a loadport, the IM needs to be fairly compact. While typical stand-alone metrology tools may have a fan filter unit (FFU) to filter out particles, condition the air temperature, and provide laminar downflow, there is generally not enough space in the IM to include this FFU, so the conditioned air is provided by the EFEM.
In some cases, the metrology instrument in the IM module may be a spectroscopic reflectometer (SR), but other types of metrology instruments may be used. Generally, the spectroscopic reflectomer measures the reflectivity of the wafer across a range of wavelengths. This information can be used to derive, for example, a film thickness of a thin film on the surface of the wafer, or the critical dimension (CD) of a device on the wafer. In some implementations, the wafer is loaded face down into the IM module, with the optics of the metrology device located below the wafer. The optics may move under the wafer to measure desired locations on the wafer. In some implementations, both the optics and the wafer may move, or only the wafer may move.
With the IM module directly connected to the process tool, wafers may be loaded into the IM module immediately after they leave the processing module. Measuring wafers immediately after leaving the processing module allows for rapid process feedback, but in many cases remnants from the processing are still present on the wafer, which may affect the performance of the metrology device. For example, in the case of an etch process, the etch gases may be absorbed by the processed wafer, and slowly leak out (outgas). For example, in the case of polysilicon etch, bromic acid (hydrogen bromate, HBrO3) is sometimes used. When wafers are measured immediately after the etch process, the etch gases may leak out into the environment of the IM module.
The introduction of process remnants, such as etch outgas, may affect the performance of the optical metrology device in the IM module. For example, gases that are used to etch silicon may etch the lens of the metrology tool or the window between the wafer and the lens, thereby changing the optical properties of the lens or window. Other gases outgassed by the wafer may condense on the surface of the lens or window which will also adversely change the optical properties of the lens or window.